This disclosure relates generally to the field of digital processing. More particularly, but not by way of limitation, it relates to a technique for stabilizing auto-exposure operations in a digital image capture device.
Modern digital video cameras use techniques to estimate how much light a scene has and adjusts the camera's exposure time to maintain the level of light collected by the camera's sensors; this is generally referred to as auto-exposure adjustment or auto-exposure control. Scenes estimated to have to little light are brightened by increasing the exposure time (allowing more light to strike the camera's sensors). Scenes estimated to have to much light are darkened by decreasing the exposure time (allowing less light to strike the camera's sensors). Changes to the exposure often trigger changes in the white balance. Even sophisticated high-end digital cameras have difficulty determining the amount of actual light that exists in a scene due to differences in the luminance of features within a scene. A scene with many low luminance objects will often be falsely identified by a digital camera as a scene with low-light even when the scene's lighting is sufficient.
Current auto-exposure algorithms also face a difficult trade-off between adjusting the light too frequently or not frequently enough. Adjusting exposure too quickly can often result in the appearance of lighting flicker. Adjusting exposure too slowly can result in poor illumination. The problem is compounded by the difficultly of distinguishing between true lighting changes and changes in a scene's composition. The lighting flicker problem can be especially troublesome when combined with a video-encoder, as encoding efficiency can be greatly impacted by quick changes in the scene's lighting (due to exposure changes).